Experts estimate that there are approximately 650,000 homeless people in the U.S. and perhaps 100 million worldwide, though accurate numbers are not available for many countries.

Ulla Beijer from the Karolinska Institute in Stockholm and colleagues performed a meta-analysis to determine the prevalence of 3 infectious diseases of public health significance -- HIV, hepatitis C virus (HCV), and TB -- among global homeless populations.

The researchers searched the PubMed, Embase, and Cumulative Index to Nursing and Allied Health Literature databases for relevant studies published between 1984 and 2012; they also searched bibliographic indices, scanned reference lists, and corresponded with study authors. They explored potential sources of heterogeneity (inconsistency) in the estimates and calculated prevalence ratios to compare estimates for homeless people compared with the general population.

Results

43 eligible studies were identified-- all but 4 from the U.S. or Europe -- with a total population of 63,812 homeless people; this was reduced to 59,736 after accounting for duplication due to overlapping study populations.

Prevalence of HIV infection ranged from 0.3% to 21.1%.

Prevalence of hepatitis C ranged from 3.9% to 36.2%.

Prevalence of TB ranged from 0.2% to 7.7%.

There was substantial heterogeneity in prevalence estimates for all 3 infections.

Prevalence ratios for homeless people compared with the general population ranged from 1 to 77 for HIV, 4 to 70 for hepatitis C, and 34 to 452 for TB.

In the U.S., homeless people had a 20-fold higher HIV prevalence, 4-fold higher HCV prevalence, and 46-fold higher TB prevalence compared with the general population.

Similar patterns were seen in most other countries with available data, including the U.K., France, Sweden, Brazil, and India; adequate information was not available for most low-income countries.

TB prevalence was higher in studies that used chest X-rays compared with other diagnostic methods.

HIV prevalence among the homeless was lower in more recent studies and higher in the U.S. than in other countries included in the analysis.

Higher infection rates were associated with prolonged homelessness and malnutrition.

"Heterogeneity in prevalence estimates for tuberculosis, hepatitis C virus, and HIV suggests the need for local surveys to inform development of health services for homeless people," the study authors concluded.

"The role of targeted and population-based measures in the reduction of risks of infectious diseases, premature mortality, and other adverse outcomes needs further examination," they added, suggesting that focusing on homeless people for detection and treatment could have a major impact on public health.

TB prevalence among the homeless was higher in countries with higher prevalence in the general population, but this was not the case for HIV or HCV, they explained in their discussion. "This result is potentially important from a public health perspective because it suggests that general population measures to reduce rates of hepatitis C virus and HIV infections might not translate into lower prevalences in homeless people," they wrote.

Although they could not identify enough studies to include in the meta-analysis, the authors noted that homeless people have also been found to have high rates hepatitis A and B, diphtheria, influenza, foot problems, and skin infections including scabies and lice.

They recommended that TB screening should involve "active case-finding," and not be restricted to symptomatic individuals accessing health services. Measures including syringe exchange programs and free condom distribution could help prevent HIV and hepatitis C.

In addition to these targeted measures, they added, "reduction of the inequalities faced by homeless people in overall social determinants of health could be part of a wider public health strategy to address infections in some countries."